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EIA-232 Information


This is one of the most misunderstood areas of RS-232. DTE stands for Data Terminal Equipment, and DCE stands for Data Circuit-terminating Equipment or Data Communications Equipment. The DTE is typically either a dumb terminal or the serial port on a computer/workstation. The DCE is typically a modem, CSU/DSU, or other piece of data communications equipment.

Where it gets confusing is when you start to talk about signal definitions and direction. For example, it's easy for someone to understand that when you transmit data, you send it out. However, when you talk in terms of the DCE, it becomes an input. This is because the specification was written from the perspective of the DTE end of the link. Another example is that the Receive Data signal is an input to DTE, but an output from DCE. See Table 1 below.



DTE Signal Direction Signal DCE Signal Direction
Bi-directional Shield Bi-directional
Output TxD Input
Input RxD Output
Output RTS Input
Input CTS Output
Input DSR Output
Bi-directional SG Bi-directional
Input DCD Output
Input SDCD Output
Input SCTS Output
Output STxD Input
Input TxClk In Output
Input SRxD Output
Input RxClk Output
Output SRTS Input
Output DTR Input
Input RI Output
Output TxClk Out Input

Therefore, a straight through "one to one" cable is all that is necessary between a modem and a standard DTE serial port. However, if you want to connect two DTE ports together, you have to simulate the existence of the pair of DCE devices, typically modems, that would normally be between the two DTE devices. This is where the null-modem device or cable comes in.  See our cabling guide for more info.


Signal State Voltage Assignments (see figure 1 below):
Voltages of -3v to -25v with respect to signal ground (pin 7) are considered logic '1' (the marking condition), whereas voltages of +3v to +25v are considered logic '0' (the spacing condition). The range of voltages between -3v and +3v is considered a transition region for which a signal state is not assigned.

RS-232 signal information

Pin Number ITU-T Number Circuit Description
1 N/A N/A Shield
2 103 BA Transmitted Data (TxD)
3 104 BB Received Data (RxD)
4 105/133 CA/CJ Request to send/ready for receiving (RTS)
5 106 CB Clear to send (CTS)
6 107 CC DCE ready (DSR)
7 102 AB Signal common (SG)
8 109 CF Received line signal detector (DCD)
9 N/A N/A (reserved)
10 N/A N/A (reserved)
11 126
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